CHANNEL LENGTH SCALING FOR FOOTPRINT COMPATIBLE DIGITAL LIBRARY CELL DESIGN
Effective GDS-based channel length scaling. A library cell is designed, and then the width of the polys is increased, and the polys and contacts are shifted in order to maintain poly-to-poly and contact-to-poly spacing. The method can be used in association with a 45 nm digital library cell. Specifically, a library cell having 40 nm polys is designed, and then the width of each of the polys is increased by 5 nm to 45 nm, and the polys and contacts are shifted in order to maintain poly-to-poly and contact-to-poly spacing. The poly lines and contacts can be shifted by starting at the center and going out radially, or by beginning at the perimeter and moving radially inward. The method can be used with any library cell design which is entirely GDS based, including, for example, 32 nm library cell design.
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Within a digital library cell, channel length scaling can be achieved by increasing the size of the poly. For example, a portion of a 65 nm library cell 10 is illustrated in
It is more difficult to achieve channel length scaling for 45 nm digital libraries than for 65 nm digital libraries.
Channel length scaling in excess of 10% may lead to unwanted threshold voltage mismatch. This large shift in the threshold voltage is due to the threshold roll up/off curve. Large channel length scaling may result in additional stress induced mobility/performance variation due to contact to poly spacing.
The positioning of contacts impacts channel stress. While positioning contacts relatively close to a channel results in low stress at the channel, positioning contacts further from a channel results in greater stress within the channel.
As contact-to-poly spacing increases, the longitudinal stress generally remains constant until the contact-to-poly spacing reaches approximately 0.14 um. As contact-to-poly spacing increases, the vertical stress increases exponentially until the contact-to-poly spacing reaches approximately 0.14 um.
The force applied by the strain nitride film is proportional to the volume and proximity of the film to the channel. The film volume and proximity is modulated by the poly-to-poly distance; the contact dimension and contact-to-poly distances; and the contact pitch.
Due to the negative effects of large channel length scaling, an alternative method for channel length scaling is needed.
SUMMARYAn embodiment of the present invention provides an effective GDS-based channel length scaling method. A library cell is designed, and then the width of the polys is increased, and the polys and contacts are shifted in order to maintain poly-to-poly and contact-to-poly spacing. The method can be used in association with a 45 nm digital library cell. Specifically, a library cell having 40 nm polys is designed, and then the width of each of the polys is increased by 5 nm to 45 nm, and the polys and contacts are shifted in order to maintain poly-to-poly and contact-to-poly spacing. The poly lines and contacts can be shifted by starting at the center and going out radially, or by beginning at the perimeter and moving radially inward. The method can be used with any library cell design which is entirely GDS based, including, for example, 32 nm library cell design.
The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof; may best be understood by reference to the following description, taken in connection with the accompanying drawings, wherein:
While the invention may be susceptible to embodiment in different forms, there is shown in the drawings, and herein will be described in detail, a specific embodiment of the invention. The specific embodiment which is illustrated and described is to be considered an example of how the present invention can be employed, and is not intended to limit the present invention to that which is illustrated and described herein.
An embodiment of the present invention provides a method of channel length scaling of 45 nm digital library cells. As shown in
There is a negative impact of asymmetry which results from poly being added to only one side of a poly element. Specifically, there is process misalignment modeling wherein misalignment occurs, and there is an impact of contact misalignment on timing. As shown, asymmetry results in an increase in timing delays.
It is general practice within the industry to develop a process where the channel length scaling is performed on the optical proximity correction (OPC) mask data. Some foundries will increase the wafer cost in order to perform this process.
For 45 nm digital library cells, discrete spacing between poly may be allowed. Optimal cell density is achieved when the minimum poly spacing is used. Based on current design rules, when poly spacing is provided as being a minimum, the contact-to-gate spacing will be at a minimum. However, the use of minimum poly to poly spacing and minimum contact-to-poly spacing, does not provide sufficient process margin for OPC based channel length scaling shown in
In addition, where OPC based channel length scaling is not currently in use, use of OPC based channel length scaling for 45 nm digital libraries will require many down stream flow changes in the design flow system to accommodate OPC based flow. For example, if commercially available place and route tools are currently in use, the transition to an OPC based system will require additional resources to support additional identification layers in order to identify which cells/devices need to be changed to longer channel lengths. Furthermore, synthesis tools, placement tools and characterization and timing engine tools will need to be integrated into the flow. Thus, transition to OPC based channel length scaling will result in complexity in tools and flow.
A method in accordance with an embodiment of the present invention begins by designing a library cell 100 having a 40 nm drawn gate.
Once the library cell 100 is designed, changes are made to the design to arrive at a re-designed library cell 140 that has a 45 nm drawn gate. The re-designed library cell 140 is shown in
While the re-designed library cell 140 provides a 45 nm drawn gate 102A rather than a 40 nm drawn gate 102 (such as is provided in the initial library cell 100), poly-to-poly spacing is maintained at being minimum, and symmetry with regard to contact-to-poly spacing is maintained (at minimum). To achieve this, the contacts are moved and the polys are shifted in the re-design, as will be described in more detail herein below.
To re-design the library cell 100 to arrive at library cell 140, the channel lengths of the library cell 140 are scaled to provide 45 nm drawn gates 102A as illustrated in
Next, the poly and contact elements of the 40 nm library cell are adjusted to fit within the 45 nm library cell by simply shifting the poly and contact elements from the positions shown in
As illustrated in
As illustrated in
As illustrated in
The method of channel length scaling of the present invention presents several advantages. Because the method of the present invention is fully GDS based, tool and flow complexity is reduced. Additionally, no significant flow or place and route tool changes are necessary relative to those required for 65 nm libraries, and no down-stream tool flow changes are necessary.
Because the metal-1 footprint is not altered, the channel length scaled chip provides compatibility for chip level placement and route tools. In addition, this GDS based approach to channel length scaling provides for interchangeability of the cells.
This channel length scaling approach results in a cell density penalty of only <<10% compared to changing to a non minimum allowed poly to poly space and using the OPC based method. The method of the present invention maintains poly-to-poly spacing at minimum. In the event it is determined that the OPC based approach to channel length scaling requires a non minimum poly-poly spacing rather than a minimum poly-to-poly spacing, the method of the present invention will provide a higher density, lower area and higher yielding channel length scaled library.
Another advantage is that better lithography based CD control is implemented resulting in reduced chip variability in cell performance.
Unlike the OPC based approach, the present invention does not impact strain engineering and carrier mobility due to contact-to-gate space variation. In addition, performance variation due to contact placement and misalignment induced mobility and IS DAT change is reduced. The contact to poly short related yield loss will be lower than an OPC based approach. Larger channel length scaling may be allowed in the present invention than allowed with an OPC based approach.
The present invention also provides for dynamic and static power reduction.
Finally, use of the GDS based channel length scaling avoids the need to work with the foundry and 3rd party OPC based tool vendors, pay additional royalties and additional wafer cost in connection with an OPC based solution.
Although the method of the present invention has been described as including the step of adjusting the poly lines and contacts by starting at the center and going out radially, the adjustment can also be made by beginning at the perimeter and moving radially inward. Although the invention has been described by utilizing a poly-to-poly spacing of minimum allowed based on the technology, any acceptable poly-to-poly spacing can be used. It should be noted, however, that a cell density penalty is provided with larger poly-to-poly spacing. Larger poly-to-poly spacing will allow for fewer legs to be used in the base cell before a poly spacer is needed to maintain the metal 1 footprint compatible layout for the cell. For example, a non minimum poly-to-poly space will allow for a base cell with twenty legs or gates, however, a minimum allowed poly-to-poly spacing will allow for a base cell with 28 legs or gates. Although the invention has been described in connection with 45 nm library cell design, it is to be understood that the method of the present invention can be used with any library cell design which is entirely GDS based, including, for example, 32 nm library cell design.
While embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications to the embodiment without departing from the spirit and scope of the present invention.
Claims
1. A method of channel length scaling in library cell design, said method comprising: designing a library cell having a plurality of polys and a plurality of contacts, wherein each of the polys are spaced away from each other, and wherein each of the contacts and polys are spaced away from each other; increasing the width of the polys; and shifting the polys and the contacts such that poly-to-poly spacing is maintained and contact-to-poly spacing is maintained.
2. A method as recited in claim 1, wherein the step of increasing the width of the polys comprises increasing the width by 5 nm.
3. A method as recited in claim 1, wherein the step of increasing the width of the polys comprises increasing the width from 40 nm to 45 nm.
4. A method as recited in claim 1, wherein the step of shifting the polys and the contacts such that poly-to-poly spacing is maintained comprises maintaining the spacing between the polys at minimum allowed space.
5. A method as recited in claim 1, wherein the step of shifting the polys and the contacts such that contact-to-poly spacing is maintained comprises maintaining the spacing between the contacts and polys at minimum allowed space.
6. A method as recited in claim 1, wherein the step of shifting the polys and the contacts such that poly-to-poly spacing is maintained comprises maintaining the spacing between the polys at minimum and maintaining the spacing between the contacts and polys at minimum.
7. A method as recited in claim 1, wherein the step of designing a library cell comprises designing a library cell which comprises metal-1 patterns, and the method comprises maintaining, and not changing, the metal-1 patterns.
8. A method as recited in claim 1, wherein the step of designing a library cell comprises designing a library cell which comprises an N-active and P-active sources and drains, and the method further comprises changing areas of the N-active and the P-active sources and drains.
9. A method as recited in claim 1, wherein the step of designing a library cell comprises designing a library cell which comprises metal-1 patterns, an N-active and P-active, and the method comprises maintaining, and not changing, the metal-1 patterns, and the method comprises changing the areas of the N-active and the P-active sources and drains.
10. A method as recited in claim 1, wherein the step of shifting the polys and the contacts comprises starting at a center and going out radially.
11. A method as recited in claim 1, wherein the step of shifting the polys and the contacts comprises starting at a perimeter and moving radially inward.
12. A library cell comprising: a plurality of 45 nm polys and a plurality of contacts, wherein the 45 nm polys are set at minimum apart from each other and wherein each contact is at minimum away from at least one poly.
Type: Application
Filed: Oct 9, 2008
Publication Date: Apr 15, 2010
Patent Grant number: 8397184
Applicant: LSI CORPORATION (Milpitas, CA)
Inventor: Richard Schultz (Fort Collins, CO)
Application Number: 12/248,677
International Classification: G06F 17/50 (20060101);